1. Field of the Invention
The invention relates generally to offshore structures. More particularly, the invention relates to offshore platforms for drilling and production operations. Still more particularly, the present invention relates to jack-up rigs with adjustable ballast and buoyancy moveably coupled to their legs.
2. Background of the Technology
A jack-up rig is a type of mobile offshore structure equipped with long support legs that are lowered to the sea floor. A jack-up rig typically includes a floating hull, drilling rig supported on the hull, and a plurality of elongate legs coupled to the hull. The hull is typically towed to the desired offshore drilling location with its legs in a raised position. Upon arriving at the desired location, the legs are lowered to the sea floor, and the hull is jacked out of the water, thereby providing a raised platform for offshore drilling and/or production operations. The hull, which supports the drilling rig, is raised above the sea surface to a desired height, thereby allowing wave, tidal, and current loads to act on the comparatively smaller legs as opposed to the larger hull and drilling rig.
When the legs of a jack-up rig are lowered to the sea floor, they are typically “preloaded” to securely drive the legs into the sea bottom. Traditionally, the preload is provided by the weight of the hull, the weight of the drilling rig and other equipment supported by the hull, and the weight of ballast water that is added to the hull. In most cases, the ballast water is pumped into ballast tanks located within the hull. The additional weight provided by the water ballast facilitates and controls the penetration of the legs into the sea floor, thereby securely setting the jack-up rig. However, the additional weight provided by water ballast in the hull increases the total load supported by the hull and the jacking systems that move the legs relative to the hull. For a hull or jacking system having a particular maximum load capacity, the added weight of the water ballast reduces the capacity available for other equipment and/or quarters on the hull.
Accordingly, there remains a need in the art for improved systems and methods for preloading the legs of a jack-up rig. Such systems and methods would be particularly well received if they offered the potential to reduce ballast loads on the hull, thereby enabling the hull to support additional equipment and/or quarters.